It is known to provide vacuum cleaners with cyclonic separating apparatus for separating dirt and dust from an airflow. It is also known to provide such separating apparatus with two cyclones arranged in series, the upstream cyclone arrangement being of relatively low efficiency and the downstream cyclone arrangement being of higher efficiency. It has been found to be beneficial if a so-called shroud is positioned at the outlet to the upstream cyclone in such an arrangement. This positioning of the shroud has been adopted in vacuum cleaners manufactured and sold by Dyson Limited.
A shroud is commonly formed by providing a wall having a large number of perforations or through-holes which communicate on their upstream side with the separating chamber of the upstream cyclone. The through-holes of the shroud thus form the outlet from the separating chamber.
A problem which may be encountered with conventional separating apparatus is that, as dirt- and dust-laden air is pulled through the shroud, eddy air currents may be set up on the inner surface of the shroud, which turbulence may cause dirt and dust to be re-entrained in the airflow or to gather on the inner surface of the shroud.